DTO pattern for data transfer in Spring Boot - Interactive Code Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to declare a simple DTO class with a private field and a getter.

Spring Boot

public class UserDTO {
    private String [1];

    public String getUsername() {
        return username;
    }
}

Drag options to blanks, or click blank then click option'

Ausername

BuserName

Cname

Duser_id

Attempts:

3 left

💡 Hint

Common Mistakes

Using a field name that does not match the getter method name.

Using underscores in field names which is uncommon in Java.

The field name username matches the getter getUsername() following Java naming conventions.

2fill in blank

medium

Complete the code to create a constructor for the DTO that sets the username field.

Spring Boot

public UserDTO([1] username) {
    this.username = username;
}

Drag options to blanks, or click blank then click option'

AUser

BString

Cint

Dboolean

Attempts:

3 left

💡 Hint

Common Mistakes

Using a wrong data type like int or boolean for a username.

Using a complex type like User instead of String.

The username field is a String, so the constructor parameter must be of type String.

3fill in blank

hard

Fix the error in the method that converts an entity to a DTO by filling the blank.

Spring Boot

public static UserDTO fromEntity(User user) {
    return new UserDTO([1]);
}

Drag options to blanks, or click blank then click option'

Auser.getUsername()

Buser.getPassword()

Cuser.getEmail()

Duser.getId()

Attempts:

3 left

💡 Hint

Common Mistakes

Passing the user ID or password instead of username.

Passing a field not present in the DTO constructor.

The DTO expects a username String, so we must pass user.getUsername() from the entity.

4fill in blank

hard

Fill both blanks to complete the DTO class with a setter and override toString method.

Spring Boot

public class UserDTO {
    private String username;

    public void [1](String username) {
        this.username = username;
    }

    @Override
    public String [2]() {
        return "UserDTO{username='" + username + "'}";
    }
}

Drag options to blanks, or click blank then click option'

AsetUsername

BgetUsername

CtoString

DtoStr

Attempts:

3 left

💡 Hint

Common Mistakes

Using getUsername as a setter method name.

Using a wrong method name like toStr instead of toString.

The setter method is named setUsername and the method overriding Object's string representation is toString.

5fill in blank

hard

Fill both blanks to create a method that converts a list of User entities to a list of UserDTOs.

Spring Boot

public static List<UserDTO> fromEntityList(List<User> users) {
    return users.stream()
        .map(user -> new UserDTO([1]))
        .collect(Collectors.[2]());
}

Drag options to blanks, or click blank then click option'

Auser.getUsername()

BtoList

CtoSet

Duser.getId()

Attempts:

3 left

💡 Hint

Common Mistakes

Using user.getId() instead of user.getUsername() in the map.

Using Collectors.toSet() which changes the collection type.

We map each User to a UserDTO by passing user.getUsername(). Then we collect the results into a list using Collectors.toList().

Practice

1. What is the main purpose of using a DTO (Data Transfer Object) in a Spring Boot application?

easy

A. To manage application configuration settings

B. To store data permanently in the database

C. To handle user authentication and authorization

D. To safely transfer only necessary data between different parts of the application

Solution

1. Step 1: Understand the role of DTOs

   DTOs are simple objects designed to carry data between layers or parts of an application without exposing sensitive or unnecessary details.

2. Step 2: Identify the correct purpose

   Unlike entities or configuration classes, DTOs focus on safe and clean data transfer, not storage or security management.

3. Final Answer:

   To safely transfer only necessary data between different parts of the application -> Option D

4. Quick Check:

   DTO purpose = safe data transfer [OK]

Hint: DTOs move data safely without exposing all details [OK]

Common Mistakes:

• Confusing DTOs with database entities
• Thinking DTOs handle security
• Assuming DTOs store data permanently

2. Which of the following is the correct way to define a simple DTO class in Spring Boot using Java records?

easy

A. public record UserDTO(String name, String email) {}

B. public class UserDTO { private String name; private String email; }

C. public interface UserDTO { String getName(); String getEmail(); }

D. public enum UserDTO { NAME, EMAIL }

Solution

1. Step 1: Recognize Java record syntax

   Java records provide a concise way to create immutable data carriers with automatic getters and constructors.

2. Step 2: Match the correct syntax

   public record UserDTO(String name, String email) {} uses the correct record declaration with fields inside parentheses and empty body braces.

3. Final Answer:

   public record UserDTO(String name, String email) {} -> Option A

4. Quick Check:

   Java record syntax = public record UserDTO(String name, String email) {} [OK]

Hint: Java records use 'record Name(fields) {}' syntax [OK]

Common Mistakes:

• Using class without constructors/getters
• Confusing interface with DTO class
• Using enum for data transfer

3. Given this Spring Boot code snippet, what will be the output when the getUserDTO() method is called?

public record UserDTO(String name, int age) {}

public UserDTO getUserDTO() {
    UserDTO user = new UserDTO("Alice", 30);
    return new UserDTO(user.name(), user.age() + 5);
}

medium

A. UserDTO[name=Alice, age=5]

B. UserDTO[name=Alice, age=35]

C. UserDTO[name=Alice, age=30]

D. Compilation error due to missing constructor

Solution

1. Step 1: Understand record instantiation and methods

   The record UserDTO has fields name and age with automatic accessor methods name() and age().

2. Step 2: Analyze the returned object

   The method creates a UserDTO with name "Alice" and age 30, then returns a new UserDTO with the same name and age increased by 5 (30 + 5 = 35).

3. Final Answer:

   UserDTO[name=Alice, age=35] -> Option B

4. Quick Check:

   Age incremented by 5 = 35 [OK]

Hint: Records have automatic getters like name() and age() [OK]

Common Mistakes:

• Forgetting to add 5 to age
• Confusing method calls with field access
• Assuming default toString format

4. Identify the error in this DTO usage code snippet:

public record ProductDTO(String name, double price) {}

public ProductDTO createProduct() {
    ProductDTO product = new ProductDTO("Book");
    return product;
}

medium

A. Missing second argument for price in ProductDTO constructor

B. Records cannot be used as DTOs

C. Method createProduct should return void

D. ProductDTO fields must be private

Solution

1. Step 1: Check record constructor parameters

   The ProductDTO record requires two parameters: a String name and a double price.

2. Step 2: Identify constructor call mistake

   The constructor call provides only one argument "Book", missing the price argument, causing a compile-time error.

3. Final Answer:

   Missing second argument for price in ProductDTO constructor -> Option A

4. Quick Check:

   Constructor args must match record fields [OK]

Hint: Record constructors need all fields in order [OK]

Common Mistakes:

• Passing fewer arguments than fields
• Thinking records can't be DTOs
• Ignoring method return types

5. You want to create a DTO that hides the user's password when sending data to the client. Given the entity:

public class User {
    private String username;
    private String password;
    private String email;
    // getters and setters
}

Which DTO definition best achieves this goal?

hard

A. public class UserDTO { private String password; }

B. public record UserDTO(String username, String password, String email) {}

C. public record UserDTO(String username, String email) {}

D. public record UserDTO(String password) {}

Solution

1. Step 1: Understand the goal to hide password

   The DTO should exclude the password field to avoid exposing it to clients.

2. Step 2: Choose DTO fields accordingly

   public record UserDTO(String username, String email) {} includes only username and email, omitting password, which meets the requirement.

3. Final Answer:

   public record UserDTO(String username, String email) {} -> Option C

4. Quick Check:

   Exclude sensitive fields in DTO [OK]

Hint: Exclude sensitive fields from DTO to hide them [OK]

Common Mistakes:

• Including password in DTO fields
• Using DTO with only password
• Confusing entity with DTO